The present invention relates to a method and apparatus for performing equalisation in a radio receiver. In particular, the present invention relates to a method of performing efficient equalisation in a GSM handset and apparatus therefor.
Mobile telephone handsets include channel equalisers which form a part of the receiver circuitry in order to compensate signal corruption caused by multi-path signal dispersion. In many digital mobile telephone systems (e.g. GSM) standards are set which, amongst other things, set minimum requirements for the equaliser of each handset intended to form a part of the system.
The minimum standard required by the equaliser is usually quite high to account for worst case scenarios (e.g. hilly environments etc.). Unfortunately, in conventional equalisers this high standard of operation requires the equaliser to consume a significantly large amount of power. During standby mode the power consumption of the equaliser contributes a significant amount of the total power consumption of the handset.
The functioning of an equaliser is complex, however it can be described, at a fairly high level, as firstly attempting to identify an equivalent digital filter which corresponds to the corruption of the digital signal sent between the transmitter and the receiver (this is called estimating the Channel Impulse Response (CIR)) caused by both multi-path dispersion and the transmitting and receiving circuitry; and then secondly using the CIR thus obtained to recover as best as possible the originally sent signal from the (corrupted) received signal.
In order to obtain the CIR estimate, the transmitter frequently (as part of each frame) transmits a signal (the midamble), which is already known by the receiver, which then attempts to match the received signal with the signal as transmitted (which, as mentioned above, is already known by the receiverxe2x80x94i.e. it is stored locally). In this process the equaliser essentially attempts to perform synchronisation between the received signal and the locally stored transmitted signal. However, as a result of the multi-path dispersion a number of different synchronisation points in time may be (at least partially) appropriate.
For example, in a typical GSM mobile station receiver, the correlation between the locally stored midamble and the received signal is measured at 10 or 11 different times with a period of time, which is equal to the length of time taken to transmit a single bit of information, between each of the different times. The strength of the correlation at each of these times is measured and this value is termed a tap coeffecient or tap. At this point time synchronisation is performed in order to identify the most relevant taps, thereafter a fixed number of 5 taps are kept for further processing. The 5 tap coeffecients derived in this way are then representative of the CIR and can be used in the second stage of the equalisation (i.e. recovering the unknown part of the transmitted signal). International Puplished Patent Application No. WO 92/11708 describes such an equaliser which may be employed in the present application.
During the second stage of the equalisation process, the equaliser in essence performs an algorithm using the received data signals together with the tap coeffecients derived in the first stage as data for use in the algorithm. The performance of this algorithm is very intensive in terms of the amount of processing power required (expressed in terms of Millions of Instructions Per Second (MIPS)).). A typically used algorithm of this type is called the Viterbi algorithm for which the amount of processing power required is proportional to 2L, where L is the constraint length in units of the amount of time taken to transmit a single bit and in conventional GSM systems is numerically equal to the number of tap coeffecients minus 1 (i.e. 4 in the GSM system).
The present invention seeks to provide a method and apparatus for performing equalisation in a radio receiver which may reduce the amount of power consumed by the equaliser under certain circuimstances.
According to a first aspect of the present invention, there is provided an equaliser for performing equalisation in a radio receiver receiving a signal from a transmitter, there being a Channel between the transmitter and the receiver having a Channel Impulse Response, CIR, the equaliser comprising CIR estimation means for generating a plurality of tap coeffecients; assessment means for assessing the tap coeffecients generated by the CIR estimation means and for outputting an assessment signal; and a processing unit, adapted to receive the assessment signal, for selecting and performing one of a plurality of different equaliser algorithms on the basis of the assessment signal, whereby when one or more of the tap coeffecients may be disregarded, an algorithm which requires less processing power may be selected and performed in preference to an algorithm which considers all of the coeffecients generated by the CIR estimation means.
According to a second aspect of the present invention, there is provided a method of performing equalisation in a radio receiver receiving a signal from a transmitter, there being a Channel between the transmitter and the receiver having a Channel Impulse Response, CIR, the method comprising the steps of estimating the CIR by generating a plurality of tap coeffecients; assessing the tap coeffecients thus generated; and selecting and performing one of a plurality of different equaliser algorithms on the basis of the result of the step of assessing the tap coeffecients, whereby when one or more of the tap coeffecients may be disregarded, an algorithm which requires less processing power may be selected and performed in preference to an algorithm which considers all of the coeffecients obtained in the step of estimating the CIR.
Preferably, the step of assessing the tap coefficients incorporates taking into account the number of tap coefficients disregarded when equalising preceding frames transmitted by the transmitter. This may be achieved by a constraint length averaging means adapted to perform a statistical analysis of the constraint length used by or at some point generated within the equaliser during equalisation of previously transmitted frames.
The step of assessing the tap coefficients preferably includes taking into account the distance between the transmitter and the receiver. This is most easily derived by using a parameter transmitted by the transmitter to the receiver together with a mobile station to base transmitting station distance estimation means for receiving such a parameter. Where applicable, an excellent such parameter is that which informs the receiver how much power is required for the receiver to transmit a signal to the transmitter should it need to do so. Furthermore, the step of assessing the tap coefficients may additionally involve taking into account the environment in which the transmitter and/or the receiver are located. This is most conveniently done by an environment analysis means for analysing a parameter transmitted by the base transmitting station which includes this sort of information. Such parameters are likely to be included in future telecommunication standards such as UMTS.
Preferably, the step of assessing the tap coefficients incorporates an additional step of modifying some of the tap coefficients which may conveniently involve setting one or more of the tap coefficients to zero when they are below a certain threshold. Preferably the threshold is variable in inverse dependence on the Signal to Noise Ratio (SNR) of the received signal, whereby the lower the SNR, the greater is the setting of the threshold.
The step of assessing the tap coefficients preferably includes performing, where possible, a constraint length reduction in which one or more end tap coefficients are chosen to be disregarded. The end tap coefficients may correspond to either the least delayed or most delayed tap coefficients. In different circumstances, where a constant number of tap coefficients may be disregarded after each tap coefficient which may not be disregarded, a quasi constraint length reduction may be performed which comprises dividing (for the purposes of executing the equaliser algorithm) the constraint length by 1+the number of tap coefficients disregarded after each tap coefficient which is not disregarded, and performing the equaliser algorithm the same number of times, separately on each of the same number of subflows of received data signals or samples, each subflow being generated by considering only every second, third, fourth, etc. sample in each subflow depending on whether one, two, three, etc. taps have been removed after each non-disregarded tap, so as to generate two, three, four, etc. subflows. In other words, where there is a spacing N between the remaining taps, equalisation is performed separately over N subflows.